The objective of the proposed research is to compare the benefit/risk characteristics of alternative direct current waveforms for ventricular defibrillation. Although all presently used waveforms achieve defibrillation, some leave the ventricles in a hypodynamic state following defibrillation. We have demonstrated this fact in the intact animal and in the isolated, metabolically working, mammalian heart. In addition, we have developed a theory that predicts different degrees of myocardial depression for different shock durations and waveforms. The theory is based on electrical breakdown of cell membranes which are dielectric in nature. According to this hypothesis, we predict that benefit/risk ratio depends on shock duration and the ratio of peak to average current (crest factor). The special isolated, metabolically supported heart preparation we have developed lends itself directly to the quantitation of myocardial depression due to defibrillating current. The isolated heart allows accurate measurement of threshold shock strength for defibrillation and shock strength for functional depression, without the interference of autonomic reflexes, which may mask waveform differences. The resistivity-matching technique that we have developed will guarantee that the isolated heart is exposed to an approximately uniform current density, thereby maximizing the reproducibility and sensitivity of the studies. We will use this preparation to test our hypothesis and to identify the type of waveform that defibrillates effectively while producing the last post-shock myocardial depression.